net:atm:fix up ENOIOCTLCMD error handling

At commit 07d106d0, Linus pointed out that ENOIOCTLCMD should be
translated as ENOTTY to user mode.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>